1. Field of the Invention
The present invention relates to a blood pressure monitoring apparatus for noninvasively and continuously monitoring blood pressure.
2. Description of the Related Art
There are many opportunities for measuring blood pressure for purposes such as a medical examination, a diagnosis of a disease, and daily physical condition check-up. However, but these measurements are generally not continuous measurements. On the other hand, in places such as an operating room and ICU for treating serious patients, continuous blood pressure monitoring is sometimes necessary. An apparatus using blood pressure estimation based on the pulse wave propagation time is conventionally known as a blood pressure monitoring apparatus for noninvasively and continuously monitoring the blood pressure.
The time (pulse wave propagation time) required for a pulse wave to propagate between two points in a living body, or the pulse wave propagation velocity obtained by dividing the blood vessel length between the two points by the pulse wave propagation time has a correlation with the blood pressure. Therefore, the blood pressure can be continuously acquired and monitored by, e.g., continuously measuring the pulse wave propagation time and applying it to an equation having a calibrated coefficient, thereby continuously calculating estimated blood pressures (e.g., Japanese Patent Application Laid-Open No. 10-66681).
To measure the pulse wave propagation time, however, pulse waves must be measured in two different locations of the body, so the measurement is complicated. Also, injuries of the patient or interferences with another medical tool sometimes make it difficult to attach sensors or cuffs for measuring pulse waves to the measurement locations. As described in Japanese Patent Application Laid-Open No. 10-66681, therefore, a general approach is to calculate the pulse wave propagation time by using an electrocardiogram (ECG) which is normally continuously measured by a biological information monitoring apparatus, and a pulse wave measured in one predetermined portion (e.g., a fingertip) of the patient.
This method has a merit in that it is unnecessary to add any dedicated sensor for measuring the pulse wave propagation time. Unfortunately, the use of an ECG in the calculation of the pulse wave propagation time has a demerit that the measurement accuracy of the pulse wave propagation time decreases. That is, an ECG is a signal which represents not a pulse wave but the electrical state change of the heart, so there is a time difference (preejection period) between the timing at which the electrical state change occurs and the timing at which the heart actually contracts to generate a pulse wave. Accordingly, the pulse wave propagation time, calculated by using the observation timing of a feature point of an ECG as a starting point, contains an error caused by the preejection period.
If the preejection period is constant, this error is easy to correct. However, the preejection period changes from one person to another, and can change occasionally even in the same person. Therefore, the accuracy improvement by the correction is limited.
A blood pressure monitoring apparatus normally monitors whether the blood pressure continuously calculated on the basis of the pulse wave propagation time falls within a normal range or not. If the blood pressure is abnormal, a more accurate blood pressure measurement is performed by using a cuff or the like, and an alarm is output if the measurement result is also abnormal.
Blood pressure measurement using a cuff is well-established as a noninvasive blood pressure measuring method, and is effective to automatically obtain a reliable blood pressure. However, this method requires avascularization, so the frequent use of the method is undesirable because the burden on a patient increases. Therefore, to determine the necessity of cuff blood pressure measurement on the basis of an estimated blood pressure, which is calculated by using the pulse wave propagation time calculated from an ECG and a pulse wave observed in one portion of the body, moderate threshold values are set to detect an abnormal blood pressure so that cuff blood pressure measurement is not performed more than necessary due to an error of the estimated blood pressure. If the threshold values are too moderate, however, an abnormality of the true blood pressure also becomes difficult to detect, and this makes appropriate threshold values very difficult to set.
To fundamentally solve this problem, the accuracy of the estimated blood pressure must be increased.